Given how tinny the roof sounds, I suspect its quite a culprit. I've done this on a previous car with great success, can highly recommend second skin ssludge as an Equiv to the under floor stuff linked. Just paint it on and leave to dry for 24hr.
Open cell foam works differently, this mat, dynamat, second skin stuff etc are all 'mass loaders' that stop the car body vibrating, and therefore making or transmitting sound. The foam stops created sound from being transmitted. Better to stop the sound at source with the mass loader I've always thought. The foam is for going to the next stage, and from stopping rattles.
Just what I have found playing previously.
Good to know the Leon can be improved so easily. It was pretty quiet when new 2 years ago, noisier now, but it could be tyre wear, as the fronts need replaced.
Avendit